Field of the Invention
The present invention relates to an engine for an outboard motor, and more particularly, to an engine for an outboard motor suitable for a model having a secondary air supply capability.
Description of the Related Art
An outboard motor mounted on a small boat and the like is configured such that an upper unit and a guide exhaust are connected to an upper part of a lower unit provided with a propeller, an engine is supported and fixed on the guide exhaust, the engine is covered by an engine cover, and a propeller is rotated and driven by the engine. An exhaust gas output from the engine is discharged to the seawater through an exhaust passage vertically provided on the side face of the engine across upper and lower units.
In such an engine for an outboard motor, a catalytic converter may be provided in the exhaust passage in order to purify the exhaust gas and comply with an exhaust gas control requirement in some cases. Furthermore, an air pump configured to supply the secondary air to the exhaust passage is provided in order to supply oxygen necessary to oxidize and detoxify hydrocarbons and carbon monoxides contained in the exhaust gas in the exhaust system.
For example, Japanese Laid-open Patent Publication No. 2010-53771 discusses an outboard motor provided with an air pump supported by an engine to supply secondary air to the upstream side of the catalytic converter in the middle of the exhaust passage. An air inlet port (air inlet hole) of the air pump is opened in a position higher than the engine inside a cowling in order to prevent water from mixing to the secondary air supplied by the air pump to the exhaust passage.
Patent Document 1: Japanese Laid-open Patent Publication No. 2010-53771
In general, an engine, an intake system, and an exhaust system of an outboard motor are housed in an engine housing covered by a cowling which is an engine cover. It is not easy to arrange accessories such as a catalytic converter or an air pump in addition to the intake and exhaust systems within a limited space of the engine housing. That is, a limitation inevitably exists in arrangement due to a relationship with neighboring components. It is difficult to compactly arrange a plurality of functional devices or components while the exhaust gas purification functionality is reliably obtained. In practice, this results in a size increase of the outboard motor.